Converted-wave migration by depth extrapolation

Recursive migration methods based on wavefield extrapolation have the advantage over Kirchhoff methods in that they accurately handle the finite-frequency effects of wave-propagation. For example, rough velocity models and triplicating wavefronts that can cause problems for Kirchhoff migrations present no difficulties for recursive methods. Biondi (2000) demonstrated these advantages for a complex 3-D P-wave dataset.

For the examples in this paper, we migrated the prestack data with a shot-profile migration algorithm, imaging the cross-correlation of upgoing and downgoing wavefields at zero-time Claerbout (1971); however, the methodology is also applicable for ``survey-sinking'' shot-geophone algorithms based on the double square root equation Claerbout (1985).

Migrating converted waves with conventional wavefield extrapolation algorithms simply involves extrapolating downgoing waves with the P-wave velocity field, and upgoing waves with the S-wave velocity field. The difficulty comes in the interpretation of common-image gathers in terms of incidence angle at the reflector.